Numerical modelling techniques for fretting fatigue crack initiation and propagation

status: publishe

Experimental and numerical investigation into effect of elevated temperature on fretting fatigue behavior

Fretting fatigue damage occurs in contacting parts when they are subjected to fluctuating loadings and sliding movements at the same time. This phenomenon may occur in many applications such as bearings/ shafts, bolted and riveted connections, steel cables, and steam and gas turbines. In this paper, the effect of elevated temperature on fretting fatigue life of Al7075-T6 is investigated using a new device for fretting fatigue tests with variable crank shaft mechanism. Also a finite element modeling method was used to estimate crack propagation lifetime in aluminum alloy, Al7075-T6 specimens at elevated temperature under fretting condition. In this method, shear and normal stresses that are caused by contact load are updated at each crack growth increment. Finally, a comparison between the experimental and numerical results is done in order to evaluate the FE simulation. Department of mechanical engineering, Islamic Azad University, Takestan Branch, Takestan, Iran The experimental results show that: (i) fretting fatigue life of the material increases with temperature up to 350°C by 180% for low stresses and decreases by 40% for high stresses, (ii) this fashion of variation of fretting fatigue life versus temperature is believed to be due to degradation of material properties which occurs by overaging and wear resistance increase due to oxidation of aluminum alloy. While overaging gives rise to degradation of mechanical strength of material and hence the reduction of its fretting fatigue life, surface oxidation of the specimens brings some improvement of fatigue behavior of the material. Metallurgical examination of the specimens reveals that temperature results in precipitation of impurities of al-7075-T6. The size of precipitated impurities and their distances gets bigger as temperature increases. This could be a reason for material degradation of specimens which are exposed to heating for longer time duration

Design of a fretting fatigue test rig with compliant springs

Fretting is a small amplitude oscillatory movement which occurs between contacting bodies who are subjected to cyclic loading or vibrations. In the contact area arise stresses -normal and tangential- causing stress concentrations and surface degradation. Fretting in combination with ordinary fatigue will reduce the plain fatigue lifetime due to the additional stresses and surface degradation. Applications susceptible to fretting fatigue are for instance connections techniques. By nature they have contacting bodies and transfer loads or vibrations. Connection techniques commonly seen in the field of fretting fatigue are lap-joints, dovetail connections and spline connections. Although mitigating or managing fretting fatigue is industrially relevant, academic research is confined and only revealed basic insights. Therefore, we developed a fretting fatigue test rig at Soete Laboratory. The paper describes the design of the test rig and points out some important features. To study the general phenomenon of fretting fatigue we selected a coupon scale test rig rather than a full scale test rig where only one application can be studied. The specimens used are one dog bone specimen and two indentation pads which make contact with the dog bone specimen. The normal force FN between the specimen and the pads is close loop controlled with a hydraulic actuator. A second hydraulic actuator controls the dynamic force Ffat in the dog bone specimen. A third force FT is introduced during dynamic loading between the dog bone specimen and the pads by means of compliant springs. The combination of these three forces: FN, Ffat, FT in the contact area gives rise to the fretting fatigue phenomenon. The test rig at Soete laboratory can be used to study fretting fatigue and examine mitigations such as surface texturing, surface work hardening, etc

Contact mechanics in fretting fatigue

This paper studies the contact mechanics in a line contact during fretting fatigue conditions. In literature one can find numerical and analytical solutions of normal and tangential stresses for a variety of loading cases. However, a unified solution valid for all loading cases during fretting fatigue conditions is not available. We present in this paper a strategy to combine existing contact mechanics theories into a unified calculation procedure. Therefore, the relevant contact mechanics theories for an idealized cylinder-on-flat contact are selected and bundled. Two clear flowcharts group the existing theories, which results in a unified strategy that can easily be implemented in a programming language. A Matlab script was programmed and calculates the normal and tangential stress distribution based on the applied forces, the geometry of the contact, the coefficient of friction and the material properties. The present theory can be used to automate the calculation of the stress distributions, or as validation of new numerical techniques. The script is modular and can be extended to calculate the lifetime of a component, by adding lifetime criteria